The present invention is generally related to controlled atmosphere incubators and, more specifically, to an improved incubator used to culture biological specimens.
Growing cell cultures in a laboratory incubator requires that the atmospheric conditions, such as temperature, humidity and gas concentrations, remain constant throughout the incubator. A common manner of humidifying the culturing environment or incubator chamber is to place a stainless steel pan of water in the bottom of the incubator. The water in the pan evaporates and, since the incubator requires a gas tight seal, the humidity level inside the incubator climbs to a level above 95% relative humidity. These high levels of humidity keep the cell cultures and their associated media from drying out during incubation. This is particularly critical when the volume of media is very small and the time required to culture the cells spans, for example, several days or more.
Although it is desirable to maintain these high levels of humidity for culturing and for fast humidity recovery after the incubator door is opened, it is not desirable to have condensate form anywhere inside the incubator. Condensate creates potential places for molds, spores and other unwanted bacteria to grow. Condensate will develop on any "cold spots" when the temperature on a surface is below the dew point of the air/gas mixture inside the incubator. Generally, incubators operate at a temperature of 37.degree. C. and at elevated humidity conditions. The dew point, for example, at 37.degree. C. and 98% relative humidity is 36.6.degree. C. Therefore, any surface inside the incubator at a temperature below 36.6.degree. C. and in contact with the air/gas mixture will condense the water from the mixture in the form of small droplets. These may then develop into pools or puddles of condensate. It is desirable for this reason as well as others to maintain all interior surfaces at a constant temperature, however, there have been some practical limits that have required less than perfect conditions.
One location within the incubator where condensate tends to form is on the inner glass door to the incubator chamber. Generally, these doors are heated to prevent condensate from forming, especially after the door has been opened. For example, electric heaters are often placed in the outside, insulated door and heat generated by these heaters is conducted, convected and radiated through the air space between the outside, insulated door and the inner glass door. Because the heat must be transferred through the air gap between the two doors, heating of the inner glass door is relatively slow and inefficient. A more direct way of heating the inner glass door is disclosed in U.S. Pat. No. 4,039,775. This patent discloses silk screened conductive elements or lines on the glass such as are commonly used in automobile window defrosters. Problems with such silk screened window defrosting elements, however, include reduced visibility through the glass door. If these lines are especially close together, visibility is significantly reduced and if the lines are placed wider apart to increase visibility, sufficient heat may not be transferred to the glass door. Also, these conductive line elements tend to eliminate condensate only along the elements themselves, or if heated to the point that condensate is eliminated on the entire glass panel, then too much heat may be generated and the chamber may be overheated. Finally, these conductive lines can be damaged by abrasion and lose their conductive and heating capabilities.
Other problems associated with the inner glass door of laboratory incubators involve the gasket which seals the door to the perimeter of the chamber opening and the mountings used to connect the glass door to the incubator. Specifically, a gas tight seal is generally accomplished using a silicone "feather" gasket mounted around the opening of the chamber with the "feather" portion of the gasket providing a seal against the inner glass door in the closed position. To maintain the integrity of the seal, the conventional method of mounting the gasket to the chamber is by using a silicone adhesive/sealant. The gasket, also generally formed of silicone, is extruded in a profile that creates a groove for the adhesive. These gaskets are difficult to clean because of their relatively complex geometry. A particularly dirty gasket may be replaced in the field by peeling the gasket off the chamber, removing the excess silicone adhesive and attaching a new gasket in the same manner as the original one. This process, however, is tedious and requires significant down time. With respect to the door mountings, hinges are generally permanently mounted to the chamber by spot welds. As these hinges may not be removed, the direction that the door swings open and closed is determined by the side of the chamber having the hinges. Field reversible doors have been an even more significant problem in water jacketed incubators since these hinge mountings must generally be placed through the water jacket portion of the incubator.
An air circulation system is also a vital ingredient in creating the correct environmental conditions for the growth of cell cultures in a laboratory incubator. Air circulation is needed to maintain temperature uniformity within the chamber and also to effectively distribute and mix the various gases, such as CO.sub.2 and N.sub.2, used to control the pH and O.sub.2 levels within the chamber. The air flow keeps the lighter gases from stratifying within the chamber and aids in the control of CO.sub.2 and O.sub.2 levels by providing air flow across the gas sensors. A blower is generally used in conjunction with a high efficiency particulate air or "HEPA" filter for circulating the air and removing contaminants from the air. The HEPA filters must be maintained at a temperature above the dew point of the air mixture to prevent condensation from developing inside the filter. This condensation can restrict or block the flow of air through the filter. Problems which currently exist with such air circulation systems include the requirement for an additional heat source to maintain the temperature of the HEPA filter above the dew point of the air mixture. Also, HEPA filters have generally been mounted in locations requiring the removal of side panels and other hardware associated with the incubator in order to access the filter for replacement. As the researcher or operator may be exposed to high voltage components when removing these incubator panels, a qualified service technician must be used for what should otherwise be a simple filter replacement procedure.
While more complicated humidifying devices may be used to control the relative humidity within the chamber, the simplest device and most common method involves placing a pan of water at the bottom of the chamber and allowing the chamber to become saturated through evaporation of water from the pan. Unfortunately, this simple method of humidification is not easily controlled and the resulting fully saturated condition more easily leads to the development of condensation within the chamber.
With regard to temperature control within the chamber of the incubator, a variance in the line voltage applied to electric components which generate heat will vary the heat output of the particular electric component. Inconsistent heat output of such incubator components as heaters and motors makes it difficult to accurately and uniformly control the temperature of the incubator chamber.
Laboratory incubators simulating biological conditions also generally include carbon dioxide sensors to regulate the amount of CO.sub.2 within the chamber and thereby simulate a specific pH or acidity level. Two general types of CO.sub.2 sensors are sensors based on a thermal conductivity detection and sensors utilizing infrared technology. With respect to thermal conductivity CO.sub.2 sensors, while these sensors are generally less costly, they are also sensitive to humidity and oxygen levels and to temperature variations within the chamber. While certain compensation systems have been proposed, these systems have not entirely solved the problems with environmental sensitivities. Infrared sensors are much less sensitive to the above noted environmental conditions. However, calibration requires the use of a tank of gas having a specific percentage of CO.sub.2.
In view of the above noted problems and deficiencies of incubators in general, there is a need for an incubator which provides a more accurate simulated chamber condition and which is more easily operated and maintained in the field by the end user.